There are numerous devices which include optical elements in which light is received or through which light is transmitted. If the received light is to be transmitted through the optical element, it is desirable to reduce Fresnel losses caused by abrupt changes in the index of refraction, e.g., when light travelling through a vacuum or air reaches the optical element.
It has long been known to provide surface layers to such optical elements, wherein the index of refraction can be varied controllably to provide antireflection properties. Such antireflection layers are typically provided at the surfaces of a variety of optical elements, and often such layers are absolutely necessary for optimum performance of the devices. The optical elements themselves may be very small, e.g., the end of an optic fiber, or may be comparatively large, for example a canopy of a high performance fighter aircraft at which radar reflections are to be reduced.
The copending application on which this continuation-in-part is based, U.S. Ser. No. 07/724,019 U.S. Pat. No. 5,164,945 deals principally with laser devices, such as contact surgical devices in which laser light is applied as a laser light flux of intense energy for ablation or vaporization of tissues. Such devices typically include a tip formed of a ceramic material, e.g., YAG, sapphire, zirconia or silica, to which laser light is provided through an optic fiber having a different index of refraction. The parent application focused principally on methods and optical structures for obtaining reduced Fresnel losses at interfaces between two elements through both of which the laser light was to be transmitted efficiently. In such applications, Fresnel reflections tend to create an undesirable hot zone near the interface where the refractive index changes abruptly. In the prior art, this was typically dealt with by cooling of the interface region. Such a solution requires additional elements in the laser optical system and tends to make the surgical tool bulky and unwieldy.
There are many other situations were Fresnel losses produce other kinds of problems.
As is well-known, the display screens of computer monitors often produce a glare when viewed in ambient fluorescent or incandescent light because of reflection from the screen to the eyes of the operator. This is typically true both for cathode ray tube-type display devices and for liquid crystal-type and plasma discharge-type devices.
Numerous filter-type elements have been designed and are marketed for placement over a visible portion of the device being observed by a user to reduce such glare. These filter-type screens work by attenuating the reflected light by passing the reflected portion of incident ambient light through a polarizing filter or a tinted (baned pass) filter. Unfortunately, light emitted from the displayed device itself is also thus attenuated, thereby reducing the visible intensity and readability of information displayed on the device screen. Such filter devices are often cumbersome and can easily become dislodged from the display screen.
Incorporating an antireflection layer in the outer surface of the display screen glass itself should provide a major improvement in the overall visual appearance of the output of the display. A graded refraction index layer, with a very narrow band rejection filter, can be designed and formed in the output surface of the display screen, to attenuate only that portion of the optical spectrum which produces the undesirable reflected glare. If this is done, the light produced by the display screen itself will not be attenuated, thereby producing an output image which is perceived to be brighter. This, when coupled with a reduction in reflected glare from the surface of the screen due to ambient lighting will produce an output image which is brighter and clearer and will therefore be easier for an operator to read. With the increasing concern nowadays over job-related stress and trauma, such a solution to reduce operator fatigue is obviously desirable.
As noted earlier, canopies of hard performance fighter aircraft also require that antireflection layers be applied to both the inside and outside surfaces. On the inside surface of the canopy, antireflection coatings are desirable to reduce glare caused by internal lighting, to facilitate observation of the flight instruments and indicators by the occupants of the fighter aircraft. The outside layer is also preferably formed to allow reduced reflection, so as to function both as a radio wave absorber (for reduction of the radar cross-section of the aircraft) and as an optical light transmitter (to enhance visibility of the outside to the plane's occupants). Considering the size and shape of the typical fighter aircraft canopy, the needed antireflection surface properties coatings and band pass filters are difficult to manufacture with known antireflection coating techniques.
Another class of optical devices requires that received light, which may be within a relatively broad range of wavelengths, be readily passed within a selected band of the received optical spectrum while being minimized in other selected bands of the same received spectrum. For such optical devices, it is desirable to be able to provide an antireflection band pass filter in the light-receiving surface of the optical element. An example of such an optical device is an infrared sensing detector which usually is fabricated from materials such as cadmium sulfide and mercury cadmium telluride, which are materials sensitive to infrared radiation in the 0.5 to 20 .mu.m wavelength range. These infrared sensors are found in a variety of infrared sensing/imaging devices, such as the sensors for heat-seeking missiles, wind shear detectors for aircraft, and infrared imaging satellites. The infrared-sensitive detector materials must be protected from the environment and are thus always placed behind protective windows and focusing lenses. These windows and lenses must be fabricated from infrared-transmitting materials which typically include quartz (silicon-dioxide), silicon, germanium, zinc sulfide and zinc selenide. The specific material is selected to ensure that only a selected portion of the received infrared spectrum is detected or imaged.
In all such devices, however, the index of refraction of the materials of the optical device is higher than that of ambient air. This necessitates the use of an antireflection layer to optimize transmission of the selected band of the infrared radiation which is to be detected or imaged through the protective window or lens. In many cases, conventionally deposited antireflection materials, such as magnesium fluoride or silicon monoxide, do not adhere well to the surface of the infrared window or lens.
The Handbook of Laser Science and Technology, at Chapter 2.2, pgs. 431-458, "GRADED-INDEX SURFACES AND FILMS", by W. Howard Lowdermilk et al, discusses a variety of known techniques for providing graded refractive index layers at or on optical substrates and provides an analysis of the physics involved. This reference mentions that graded-index surfaces and films for broad band antireflective (AR) property have important applications in solar energy collection systems, antiglare display cases, and for optical recording discs on which a laser recording is to be made. The reference discusses how to generate and study non-absorbing graded surfaces and films for visible and near-IR light to increase its transmission through a substrate material. The reference states that such an antireflective treatment is typically obtained by chemical leaching and etching of a light-receiving surface of an optical substrate to remove certain components of the substrate material to create a porous, skeletonized, surface region which has pores of a dimension smaller than the wavelength of the light that is to be transmitted. Consequently, such techniques are limited to optical substrate materials with leachable components. Lowdermilk et al also discusses alternatives, including one in which a non-crystalline multicomponent, inorganic oxide film is deposited by a so-called sol-gel process, the film then being itself etched by chemical leaching and/or etching. Another alternative discussed is ion implantation into plastic surfaces, with subsequent etching of the ion tracks to produce a microporous surface region. Yet another proposed alternative is to form a so-called "Moth's Eye" type surface which has distributed fine protuberances on a light-receiving portion thereof. The last alternative discussed by Lowdermilk, at p. 454, is to provide an antireflection coating by adding a surface layer and then applying a laser to generate therefrom a laser-induced damage of the coating layer.
As will be readily perceived, leaching, etching, applying an added-on layer (which itself may thereafter by leached or etched), and other variations of such known techniques, all have significant limitations. Such limitations relate not only to the choice of materials, problems associated with producing very small or very large uniformly graded refraction index regions, high cost, and the risk that for precisely dimensioned elements such as optical lenses there will be a deleterious effect on the focusing capability of the optical element and hence of a more complex device utilizing such an optical element.
There is, therefore, a clear need for optical elements which have a region of non-abruptly index of refraction to minimize losses when light is received or to facilitate absorption or transmission of selected wavelengths of the received light, and for methods which enable this to be realized without physically damaging the light receiving surface and which do not significantly alter the geometry of the surface itself.
The present invention specifically addresses these needs, and is described fully with reference to drawing figures illustrating exemplary structures and geometries. It should be appreciated that the following description is not intended to be limiting, and it is expected that persons of ordinary skill in the art upon reading and understanding this invention will be led to make obvious modifications thereto for specific applications.
As previously noted, the parent application focused principally on the transmission of laser light into and through optical elements in a system utilizing laser light. It is specifically noted that, other than the fact that the structural forms and methods discussed therein pertained principally to uses of laser light for surgical applications, they are readily adaptable to light of any selected wavelength or combinations thereof. In other words, while laser light is coherent light, unlike light comprising a relatively wide range of wavelengths, the basic solution of providing a region of non-abruptly varying refraction index at a selected surface of an optical substrate or device is the same as in the parent application. For this reason, the teaching of the parent application, insofar as it is relevant in these regards, is expressly incorporated herein by reference.